Geosphere Highlights

Exploring Earth: From Surface to Sea

New GEOSPHERE science posted online 19 April 2012

Boulder, CO, USA – Five new Geosphere articles posted online today include additions to themed issues: "Exploring the Deep Sea and Beyond"; "Seeing the True Shape of Earth's Surface: Applications of Airborne and Terrestrial LiDAR in the Geosciences"; and "Geodynamics and Consequences of Lithospheric Removal in the Sierra Nevada, California." Locations studied: the Sierra Nevada, California; the San Juan volcanic field, Colorado; the western Alaska continental margin: Kodiak to Unimak; Pyramid Lake, Nevada; and the Appalachian fold-thrust belt, Pennsylvania.

Abstracts for these and other GEOSPHERE papers are available at http://geosphere.gsapubs.org/. Representatives of the media may obtain complimentary copies of GEOSPHERE articles by contacting Kea Giles at the address above.

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The San Juan volcanic field in southwestern Colorado, USA, is one of the largest examples of silicic volcanism in the world. The volcanic rocks on the surface have been studied in detail for several decades, although related rocks that exist at depth and were never erupted have not been well-studied. Benjamin J. Drenth and colleagues use a combination of gravity and seismic geophysical methods to study the subsurface beneath the volcanic field. They image large igneous intrusive complex to estimate its shape, thickness, and volume. These results provide new constraints on the development of the volcanic field and related buried rocks.

The western Alaska continental margin has a history of destructive earthquakes, some of which produced trans-Pacific tsunamis that inundated the U.S. mainland's west coast. These earthquakes occur when the interface between the continental upper plate and the under-thrust oceanic lower plate rupture. Three great earthquakes were recorded at multiple seismometers in the past 70 years. Their aftershock distribution approximates the extent of rupture. Roland von Huene and colleagues examined the geologic character of these ruptures to understand them better. Earthquake ruptures run through oceanic sediment which was carried on the ocean crust to their present position beneath the continent during the past 10 million years. By reconstructing the 10-million-year history of geologic processes along the continental margin, they were able to infer the types of these deeply buried ruptured materials. They found that differences in material correlate with individual earthquake ruptures. They also found that ridges in the Gulf of Alaska with volcanoes up to 3 km high intersect and rupture the continental slope topography. This geology indicates that the ridges and volcanoes most likely extend under the continent. The inferred deeply buried ridge locations occur where the ruptures of two great earthquakes on either side ended. This indicates that under-thrust ocean relief probably blocked further propagation of earthquake rupture. At other under-thrust volcanoes, earthquake rupture began, indicating that high relief on the rupture plane can also trigger earthquakes, as has been observed elsewhere. If under thrust ridges and volcanoes have controlled earthquake ruptures in the past, they may constrain and initiate future earthquakes. Such knowledge can help efforts to anticipate where the next great earthquake and tsunami may occur. The submarine Alaska continental slope is locally unstable, and huge landslide scars up to 45 km wide are observed. These may also be involved with tsunami generation, but data are insufficient along the Alaskan margin to understand their role. The tsunami of 1946 that damaged not only its Alaskan source region but also Hawaii and Pacific Islands to Antarctica is proposed to have involved submarine land sliding during this earthquake. This study helps determine where monitoring instruments and better marine geophysical data along the vast Alaska continental margin can help in the quest to anticipate future earthquake hazards that can affect the mainland U.S. west coast.

Over the last 120 years, the level of Pyramid Lake has dropped by about 20 m due to upstream water use and diversion. In response, the inflowing Truckee River incised its bed and widened its channel, causing the river to become steeper, straighter, and smoother. Kenneth D. Adams of the Desert Research Institute uses aerial photographs and LiDAR topographic data to track this incision and the formation of stream terraces along the lower 15 km of the river, beginning in 1938 and continuing to the present. The channel was at its steepest as the lake reached its historical lowstand during the 1960s, which corresponded to the highest sediment transport rates. Since that time, lake level has rebounded and the channel has grown less steep, decreasing sediment transport rates. Changes in these rates likely reflect the degree to which the channel is out of equilibrium with respect to discharge and sediment supply. The total volume of sediment removed from the lower Truckee River since 1891 is approx. 60,000,000 cubic meters, which if spread across the flat floor of Pyramid Lake would amount to a layer about 25 cm thick. This relatively rapid deposition has caused a temporary increase in long-term sedimentation rates in the lake. Extrapolating to larger spatial and longer time scales, the evacuation of large river trenches cut into the floors of pluvial lake basins was probably a rapid process as the lakes receded at the end of the Pleistocene, which caused large increases in sedimentation rates in the receiving basins.

The Sierra Nevada batholith of California is an extensive belt of granitoid rocks that developed as a result of subduction along the margin of western North America over the course of more than 100 million years. Although the Sierra Nevada has been a subject of interest and much geologic study going back to the days of the California Gold Rush, there remain outstanding questions regarding the petrogenetic growth of the batholith. By comparison with the southern Sierra Nevada, we understand relatively little about the timing of magmatism and the geochemical development of the smaller, patchier northern batholith. In this study, M.R. Cecil and colleagues sample granitoid intrusions across two range-perpendicular transects. They then determined crystallization ages, geochemical signatures, and isotopic compositions of the granitoids in order to identify spatial and temporal trends in batholith petrogenesis. Isotopic signatures in the northern granitoids are more primitive than those recorded in the southern batholith, reflecting emplacement into more juvenile lithosphere. The major and trace element geochemistry of the northern granitoids, however, is very similar to those in the southern Sierra, suggesting that emplacement into juvenile, less fertile lithosphere does not preclude volumetrically significant magmatism. Additionally, the authors recognize diagnostic trace element patterns indicative of the development of a dense, mafic root in conjunction with granitoid genesis. Available geophysical data clearly shows the absence of such a root at the base of the crust in the northern Sierra. It is therefore suggested that the mafic root was removed, perhaps in response to the onset of Miocene volcanism in the area.

Peter B. Sak and colleagues present a kinematic model for the sequential development of the Appalachian fold-thrust belt (eastern U.S.) across a classic transect through the Pennsylvania salient. They use new map and strain data to create a balanced geologic cross section from the southern edge of the Valley and Ridge Province to the northern Appalachian Plateau. This region of the central Appalachian fold-thrust belt is an ideal location to illustrate the incorporation of strain data in balanced cross sections, because it cannot be balanced without quantifying grain-scale strain. Sak and colleagues use a sequentially restored, balanced cross section to show how layer-parallel shortening (LPS) is distributed above and ahead of thrust and fold shortening and constrains the geometric and kinematic evolution of a passive roof duplex. By combining line length and area balancing of a kinematically viable cross section with LPS estimates in both the Valley and Ridge Province (20%) and Appalachian Plateau (13%), they document the total magnitude of shortening in both the folded cover sequence and the duplexed lower layer of the fold-thrust belt.